Electrowetting is a robust phenomenon in which the properties of both the insulating and the hydrophobic layers are critical. Substantial activities have been aimed at optimizing the properties of these layers in order to minimize the voltage required for water contact angle reduction and contact angle hysteresis. At the same time, the materials used should be chemically inert and stable in order to ensure reproducibility and a long lifetime.
Such materials have been approached by using either an intrinsically hydrophobic insulator or by covering hydrophilic insulators with a thin hydrophobic top coating. Frequently, thin layers of amorphous fluoropolymers or hydrocarbon polymers (such as Parylene-C or Parylene-N) have been used. However, the dielectric constant of such hydrocarbon and fluoropolymers are low; for example, the dielectric constant of Teflon AF™ is ˜1.9 and the dielectric constant of Parylene-C™ is ˜3.1. Popular hydrophilic inorganic insulator materials include SiO2 [see, Jones et al., Langmuir 19:7646 (2003); Yoon and Garrell, Anal. Chem. 75:5097 (2003); Moon et al., J. Appl. Phys. 92:4080 (2002); Huh et al., J. Am. Chem. Soc. 125:14678 (2003); Cho et al., J. Microelectromech. Syst. 12:70 (2003)] and SiN [see Moon, et al., ibid; Acharya et al., Appl. Phys. Lett. 83:4912 (2003); Krupenkin et al., Appl. Phys. Lett. 82:316 (2003)]. In combination with a hydrophobic top coating, inorganic insulators perform well as electrowetting substrates.
Applications of the electrowetting phenomena are varied. A sandwich design consisting of two parallel substrates with the liquid confined in between has become standard. One of the substrates contains an insulated electrode required for liquid actuation; the other substrate consists of a homogeneous electrode that provides electrical contact to the liquid. A variety of applications have been envisioned which take advantage of the electrowetting effect. The focal length of a liquid lens can be tuned by adjusting its shape by changing the contact angle of sessile droplets via electrowetting. This allows for the design of optical systems with variable focal length that can be addressed purely electrically, as first described by Peseux and Berge [see, Berge and Peseux, Eur. Phys. J. E 3:159 (2000)]. Examples of such devices can be found in, for example, EP 1,166,157 and US Patent Application No. 2006/0126190.
Electrowetting-based reflective displays involve laterally confining an oil droplet, containing dissolved dye, to a square pixel. Upon applying a voltage, the oil film ruptures and contracts into one corner of the pixel that can be predefined by a passive chemical wettability pattern. Other applications of this principle include switches, latching relays, optical shutters, and micropumps [see, Lee and Kim, J. Microelectromech. Syst. 9:171 (2000); Yun et al., J. Microelectromech. Syst. 11:454 (2002)].
One critical materials parameter for the insulator is its dielectric strength, or the electrical breakdown field strength EBD, which limits the minimum thickness of the insulating layer that can be used without breakdown voltage. Dielectric breakdown occurs at UBD=Ecd, where Ec is a constant which is unique for each material, and directly proportional to the energy gap for conduction in the material, and UBD is the applied voltage to the dielectric layer. The voltage required to achieve a desired variation of the contact angle Δcos θ is given by [see, Seyrat and Hayes, J. Appl. Phys. 90:1383 (2001)] U(Δcos θ)=(dσlv Δ cos θ/∈0∈d)1/2, where d is the thickness of the insulator, σlv is the interfacial energy (liquid-vapor) of the liquid on the surface, ∈0 is the permittivity of free space (8.854×10−12 C2/N m2), and ∈d is the dielectric constant of the insulating layer.
Another critical materials parameter is the reliability of the wetting property of the hydrophobic layer. Reliability is the ability for the coating to keep its initial physical properties after a set of tests. For a liquid lens device for example, reliability is evaluated with the measurements of the hysteresis, wave front error or stability of the driving voltage. Usually these tests include storage of the device at elevated temperature and ON/OFF tests (the device is alternatively turned ON or OFF for thousands or millions of times).
As can be seen from the preceding relationships, the potential required for a certain Δcos θ for a liquid can be decreased by one or more of, decreasing the thickness, decreasing the interfacial energy, and/or increasing the dielectric constant of the insulator. Current solutions using hydrocarbon or fluoropolymer insulating layers are limited due to their low dielectric strengths, necessitating thick insulating layers, typically on the order of 1 μm. Such thick layers also require higher operating potentials to achieve the desired surface effects.
Therefore, there exists a need in the art for insulating materials which address each of these material properties. Materials with higher dielectric constants in combination with lower interfacial or surface energies and high dielectric strength (or high electrical breakdown field strength EBD) would enable the electrowetting effect with increasingly thinner device architectures. In addition, the use of such thinner layers would also allow the use of lower applied potentials, each of which are highly desirable in electronic devices, and impact such considerations as device size and battery lifetime.
However, there are very few materials combining both suitable dielectric and reliable hydrophobic properties within a broad thickness range, especially when thickness is below one micrometer.
The present invention solves this problem and allows for the use of reliable and lower driving voltages in, for example, variable focal length devices, by carefully designing a stack of at least two layers, one of the layers being a hydrophobic layer arranged on top of the overall coating, in contact with the liquid, wherein the layer presenting the lowest dielectric strength, usually the hydrophobic layer, is chosen with a given thickness such as it has a negligible contribution to the overall capacitance and driving voltage, therefore enabling to choose an hydrophobic layer with the highest reliability, keeping for the overall coating a high dielectric strength.